Radiation sensitive semi-conductive layer of amorphous selenium



Nov. 25, 1958 M. PLOKE ET AL 2,862,126

RADIATION SENSITIVE SEMI-CONDUCTIVE LAYER OF AMORPHOUS SELENIUM FiledAug. 28, 1953 3 Sheets-Sheet 1 Fay] J72 yen to rs flan/red fl elierfiarzzir P101? Nov. 25, 1958 M. PLOKE ET AL 2,862,126

RADIATION SENSITIVE SEMI-CONDUCTIVE LAYER OF AMORPHOUS SELENIUM FiledAug. 28, 1953 3 Sheets-Sheet 2 I I H" States atent 2,862,126 PatentedNov. 25, 1958 This invention relates to a semi-conductive layerpreferably consisting of selenium which has been evaporated in vacuum ata temperature below 125 F. as disclosed in the German Patent 898,641.

Semi-conductive layers consisting of amorphous selenium have hithertobeen used for measuring of radiation only. Merely for quantitativeindication of the presence of X-rays, semi-conductive layers consistingof amorphous selenium were used which had been made sensitive for X-raysthrough a prolonged special treatment of the selenium. Said specialtreatment consisted at least partially in transformation of theamorphous character of the selenium into the crystalline condition,whereby a selenium was obtained which had a low ohmic resistance. Suchselenium resistant cells known as Fiirstenau Roentgen radiationintensity measuring devices show too great an inertia, fatigue andageing in connection with X-rays. The reason'for these phenomena isconnected with the crystalline intercalation'of amorphous seleniumlayers. The required inertia-free receptivity for X-rays, is due to thefact that, the X-ray electric effect, contrary to the radiation-electriceffect, depends upon an intra-atomic process for the inception of whichthe small amountof energy in the radiation is insuficient. Therefore,the radiation sensitivity is, according to earlier perceptions, coupledwith the crystalline condition of the matter even if in small tracesonly.

In order to enable the carrying out of X-ray measuring and X-ray imagestrengthening free from inertia, fatigue and ageing phenomena; it issuggested, according to the present invention, to employ asemi-conductive layer consisting of amorphous selenium as an X-raysensitive semi-conductive layer. The formation of crystalline seleniumwhich, according to the above explanations, is the cause of theestablished inertia and ageing, is avoided when the amorphous seleniumlayers are evaporated under avoidance of such crystallization, in vacuumupon a support which is maintained at a temperature below 125 F.

It is true that such cold-evaporated layers are extremely insensitive tolight as distinguished from layers which have been evaporated on asupport having a temperature of, for example, 175 F. Saidcold-evaporated layers have the adidtional advantage that theirresistance in the dark is one to two units higher than that ofhot-evaporated layers, which is of essential importance for their use asa charge feeding layer.

Another not before recognized quality of the amorphous selenium layersin connection with X-rays is that such layers do not show eitherinertia, fatigue or ageing. It has been established that initiation andtermination periods of the change in conductivity at a sudden change inthe intensity of the X-ray radiation are of the order of 10 seconds.

In the drawings,

Fig. 1 is a diagram showing the relation between X-ray conductivity andthickness of a selenium layer according to the invention,

Fig. 2 is a perspective view, partly in section, of a longitudinal cellcomprising an amorphous selenium layer,

Fig. 3 is a perspective view,-partly in section, of a highohmicX-rayresistance cell according-to the invention,

in relation to the layer thickness.

Fig. 4 shows an X-ray image iconoscope according to the invention,

Fig. 5 shows the invention applied to a conventional orthicon tube,

Fig. 6 shows a cross sectional view of a modified construction andarrangement of the semi-conductive layer and support of the invention,and

Fig; 7 shows a modified arrangement of the construction shown in Fig. 6.

In order to achieve that the X-ray sensitive element which, according tothe invention, consists of a layer of amorphous selenium, has a highsensitivity for X-rays, said layer has to be applied in relatively greatthickness. The fact is, that experiments have shown that contrary tothis, the earlier light sensitive semi-conductive layers of seleniumwhich were applied in a thickness from 1 to 10 microns have very lowX-ray sensitivity. Furthermore,'it has been established tht the X-rayconductivity of the amorphous selenium layers is dependent upon thelayer thickness to an extremely high degree. Fig. 1 shows theestablished X-ray conductivity coefiicient am For the investigation agreat number of so-called longitudinal cells were produced withamorphous selenium layers of different thickness in accordance with Fig.2. A glass plate 1 served as carrier for these layers. The amorphousselenium layer 2 is disposed between two evaporated aluminum electrodes3 and 4. By means of a battery 5 and a galvanometer 10, a circuitisclosed. From the current and the imposed voltage the conductivitychange in the cell under X-ray radiation is calculated and therefromalso the X- ray conductivity coefiicient oz in a a 1 82 cm. mr./sec.

(mr.=milli-R6ntgen=dosage unity). The X-ray radiation was obtained as a60 kv. continuous radiation from an X-ray tube with a' olframanti-cathode. The X-ray conductivitycoefiicient is thereby related tothe received dosage effect in mr. per second. No appreciable dependence.of the X-ray conductivity coeificient upon the hardness of radiationexists between 50 and kv. The dependency of the X-ray conductivitycoefficient upon the thickness of the layer runs approximately parallelwith the absorbed X-ray radiation in the selenium layer. With theradiation employed, the highest X-ray conductivity coefiicient isobtained at a thickness of the selenium layer of about 300 microns and,therefore, it is not advisable to make the layers thicker than 300microns.

X-ray sensitive amorphous selenium layers may be used in a variety ofapparatus, two embodiments being described below. Fig. 3 shows aperspective view, partly in section, of a high-ohmic X-ray resistancecell accord ing to the invention. This cell is produced by precipitatingan amoiphous selenium layer 6 of a thickness of, for example, 200microns on a metal support plate 7. A metal layer 8 penetra-ble by X-rayradiation is placed on said selenium layer as a counter-electrode. .Aselectrodes, materials are used which do not react with selenium such as,for example, aluminum, nickel, lampblack, graphite, or carbon. With thetwo metal electrodes '7 and 8 a circuit is produced over the X-raysensitive layer and the battery 9, an ammeter 10' being connected insaid circuit which shows a current dependent upon the X-ray in tensitywith which the cell is irradiated.

Fig. 4 shows an X-ray image iconoscope, the. semiconductive layer ofwhich is formed according to the in vention as an X-ray sensitiveamorphous selenium layer,

so that the X-ray image may be scanned in a direct way, i. e, withoutlight transformation. The iconoscope has a bulb 11 of glass'or aluminumwhich for the purpose of less absorption of the X-ray image isspherical, so that it may be produced with less wall thickness. For thebulb, or the image window, through which the X-ray image penetrates tothe amorphous X-ray sensitive selenium layer, preferably a materialhaving a lower absorption coeflicient than glass is used, for example,berryllium glass, or so called Lindernann glass. The screen electrodehas a conductive signal plate 12 which is transparent for X-rayradiation. As a signal plate, for example, a thin aluminum sheet mayserv e. Said signal plate carries the X-ray sensitive seleniumlayer lfi.,The 'experiments with X-ray image iconoscopes have shown that asuitable thickness of the amorphous selenium layer 13 is between 50microns and .300 microns. ,'l ."he signal plate and theiX-ray sensitiveselenium layer may also be applied directly to the tube bulb.

Over the operational resistance 14 the signal plate is supplied with abiasing potential. According to the thicknessof the amorphous seleniumlayer, said biasing potential maybe- 600 v. orlrnore. ln order tointerrupt the induced conductivity producedby the scanning ray, it maybe advisable toprovide the free surface of the amorphous selenium layerwith a mosaic 25 which is impenetrable for the electrons ofthe scanningray. The load images produced on the free selenium surface throughchange of resistance in the X-ray sensitive selenium layer are suppliedto the image amplifier 16 through the scanning by means of the. electronray in a known way over the coupling condenser 15. An accelerationelectrode 17 suitably comprising a fine mesh net or metal ring isdisposed betweenthe signalplate 12 and the scanning aperture 24 of theray system. In the neck of the glass bulb suitable elements are providedforproducing an electron ray bundle. They consist of a cathode 18 with aheating wire 19, the Wehnelt cylinder 20 and the anode 21. Throughthe'scanning aperture 24, the electron ray is finely restricted in orderto'improve the resolving power of the iconoscope. Bartspfjhe innersurface of the glass bulb are coatedwith .a conductive material so as.to deflect. charges onthe glasswall. Since the X-ray images reach theselenium layer in their natural size, a good dissolution capacity canbeobtained with the iconoscope. Due to the largedeflecting amplitudes, itis necessary to provide for particularly good magnetic screening. 7

Besides the function as an iconoscope with high scanning ray velocity, afunction with orthicon electron optics is also possible as shown in Fig.5. ,In this Figure the X-ray sensitive layer 12 with its support 13 ismounted in the envelope of a conventional orthicon tube 11a which issurrounded by the customary longitudinal focusing coil 23a, while inplace of the coil 22 employed in Fig. 4 there is used with the orthicontube 11a an adjustment coil 22a.

In order to make the selenium more sensitive or, in other words, todecrease its electrical inertia, heavy metals or heavy metalselenium-compounds may be mixed with the selenium such as, for example,lead or leadselenide. Lead selenide is known as a high-ohmic lightsensitive substance.

The proportion of said heavymetals in the amorphous selenium layershould be variable from minimum amounts to the stoichiometric relationofthe corresponding selenide in order to obtain the most advantageouscomposition for the responsiveness of the resistance layer to X-raysunder all conditions.

Within the framework of the invention also the useability of saidforeign ingredients transversely to the maximum extension of the layeris included, for example, so that the side of the layer opposite to theX-rays consists of pure selenium, while the side of said layer facingthe X-rays has an increasing proportion of foreign ingredients orselenides 13a, respectively as shown in Fig. 6. .Among the selenideshaving advantageous photoelectric qualities, silver selenide occupies aprominent position.

With regard to the employment of the cells as resistance scanners, itmay also, under certain circumstances, be suitable to enrich the sideopposite the X-ray with foreign metals 13b (Fig.7) since. the sidefacing the scanning ray, that is, the side opposite the X-ray, must havethe lower conductivity. Said two considerations may be combined'if thescanning takes place from the side of the X-ray radiation. With regardto the surface activation of selenium layers with foreign metals, it maybe advisable to apply thesame through cathode spraying in air whereby,in addition to the interior photo effect, an arrest layer effect occurs,which influences the photoelectric qualities of the, cells in anadvantageous direction. As ,an additional mixing. ingredient for theselenium layer, tellurium should talsofbe mentioned, which, as is wellknown, reduces fatigue phenomena of thephoto-eifect in selenium. i

In contrast tothe known arrest layer cells of selenium, the X-raysensitive photoelement,accordingto the inventionhasthe advantage or,high ohmic resistance which is of advantage, particularly with regard tothe use for the resistance. scanner.

What we claimv is:

.1. A semi-conductive element, comprising a radiation sensitivesemi-conductive layer ofamorphous selenium having a thickness of aboutZOO-microns, said layer having 'beenformed by evaporation in vacuumunder avoidance of crystallizationat atemperature below F. to make itsensitive for X-rays, and foreign materials having been added to saidamorphous selenium which serve for. sensitivi zing, e. .for reducingthe. electric inertia of said. selenium.

2.,An element as.in claim 1, in which said foreign materials compriseheavy metals.

3. An element as in claim- 2,.in which the proportion of saidheavymetalsisvariable from minimum values to thest ichiometric relationofthe corresponding selenide.

4. An element as in claim 1 in which said foreign materials compriseheavy metal compounds with selenium.

5. An element as in claim 1, in which the proportion of foreigningredients in theselenium layer is variable transversely to its maximumextension in such a way that the side of the layer opposite to theX-rays consists of pure selenium,.while the side of said layer facingsaid X-rays-has an increasing proportion of said foreign ingredients.

6. An element as in claim 1, in which said foreign materials compriseselenide of silver.

7. An element as in claim 1, in which the side of the selenium layeropposite to the X-rays is enriched with said foreign materials, whilethe side of said selenium layer facing the X-rays has less proportion ofsaid foreign materials.

8. An element as in claim 1, including a proportion of tellurium in saidselenium layer in order to avoid fatigue phenomena.

9. A photoelectric elementcomprising a semi-conductive layer consistingof amorphous selenium formed by evaporation'in vacuum under avoidance ofcrystallization at temperatures below 125 F. and having a thickness ofabout 200 microns so as to render theresistance of the layer sensitiveto X-ray radiation, and large area electrodes in contact with said layerat both sides of said layer, said electrodes consisting of materialwhich does not react with selenium and at least one of said electrodesbeing penetrable by X-rays.

References Cited in the file of this patent UNITED STATES PATENTS1,491,040 Hart Apr. 22, 1924 2,010,712 Coolidge Aug. 6, 1935 2,569,872Skehan et al. Oct. 2, 1951 2,608,611 Shiver Aug. 26, 1952 2,654,853Weimer Qct. 6,1953

1. A SEMI-CONDUCTIVE ELEMENT, COMPRISING A RADIATION SENSITIVE SEMI-CONDUCTIVE LAYER OF AMORPHOUS SELENIUM HAVING A THICKNESS OF ABOUT 200 MICRONS, SAID LAYER HAVING BEEN FORMED BY EVAPORATION IN VACUUM UNDR AVOIDANCE OF CRYSTALLIZATION AT A TEMPERATURE BELOW 125*F. TO MAKE IT SENSITIVE FOR X-RAYS, AND FOREIGN MATERIALS HAVING BEEN ADDED TO SAID AMORPHOUS SELENIUM WHICH SERVE FOR SENSITIVIZING, I. E. FOR REDUCING THE ELECTRIC INERTIA OF SAID SELENIUM. 